FIG. 8 is a partial sectional view illustrating one example of a thermal head, and FIG. 9 is a partial plan view illustrating the layout of a heating resistor and electrodes.
As illustrated in FIG. 8, a partial glaze layer 2 having a convex shape and made of glass, for example is disposed on an insulating substrate 1, such as a ceramic substrate. Further, a common electrode, a comb-shaped electrode extending from the common electrode, and a plurality of individual electrodes (described below with reference to FIG. 9), those electrodes being each made of an electrically conductive material, e.g., gold, are disposed on the partial glaze layer 2. Still further, a heating resistor 3 made of ruthenium oxide, for example, is linearly formed over the individual electrodes and the comb-shaped electrode on the partial glaze layer 2 (see Patent Document 2 as one example of the thermal head including the partial glaze layer).
In addition, an electrically insulating protection film 4 (hereinafter referred to as an “insulating protection film 4”) made of a PbO—SiO2—ZrO2 based glass material, for example, coveres substantially over the entire surface of the insulating substrate 1, including the partial glaze layer 2, the heating resistor 3 and the electrodes. A sheet of thermal paper 8 serving as a printing medium is conveyed while it is pressed by a platen roller 7 against the insulating protection film 4 so that a color is developed with heat generated by the heating resistor 3 and then transferred through the insulating protection film 4.
Moreover, as illustrated in FIG. 9, the color development in the thermal paper 8 is performed by supplying a current to the heating resistor 3 so as to heat individual heating resistor elements in units of one dot, which are each present between the comb-shaped electrode 6a extending from the common electrode 6 and one of the individual electrodes 5. Thus, the insulating protection film 4 serves to ensure mechanical protection and electrical insulation, and hence the insulating protection film 4 is required to have mechanical strength and electrical insulation performance in excess of certain levels.